Few things can be as aggravating as an oil furnace that shuts down almost immediately after firing, particular right after cold weather has arrived. If your furnace doesn't start at all, at least you have the comfort of knowing without a doubt that something is wrong. But a furnace that simply shuts off after a few minutes -- this is officially referred to as "short-cycling" -- can make you wonder if you have a problem or not. If your oil furnace is short-cycling, chances are you do have a problem. A number of things could be to blame.
Perhaps the most common cause of short cycling in oil furnaces is a dirty filter. A dirty filter doesn't allow enough hot air to get through. This can overheat your furnace, causing it to repeatedly shut down shortly after starting. Fortunately, this is also an easy problem to fix. You can clean or replace a filter, depending on whether your furnace takes reusable or disposable filters. It is generally a good policy to check your filters about every few months to keep your furnace running smoothly.
Registers blocked by furniture or other objects may cause your furnace to repeatedly shut down. Like dirty filters, blocked registers do not allow air to circulate well, leading to overheating and short-cycling. Some homeowners may close some heat registers in a home, perhaps hoping to reduce heating costs or to direct heat only to specific parts of the house ... but if 60 percent of your home's registers are closed or blocked off, they may be the cause of your furnace's short-cycling.
Thermostat settings could be to blame for short cycling; if your thermostat's thermostat setting is too low, it's possible that it takes your furnace only a few minutes to raise the temperature in your house to the reach the thermostat threshold temperature. The location of the thermostat can also cause issues in some homes: If your thermostat is too close to a vent or register, the hot air blowing from the register can cause your furnace to shut down if the air blows directly to the thermostat, because the thermostat thinks your house is warmer than it is. To test this potential cause, tape a small cardboard box over the thermostat and see if this affects the length of your furnace cycles.
Short-cycling may be caused by more serious problems, such as an oversize burner that is producing heat too fast for the blower fan to extract it. The blower fan itself may be dirty, blocked or broken. An overheating heat exchanger could be to blame. If the heat exchanger gets too hot, the burner should automatically shut off to prevent damage.
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